PROJECT SUMMARY Systemic inflammation is believed to contribute to the damaging health effects of psychological stress. In the laboratory setting, a single acute bout of socioevaluative stress leads to a delayed elevation of pro- inflammatory cytokines in healthy adults. However, the biological mechanisms by which psychological stress is transduced into inflammation is not well understood. A cellular component known to contribute to stress signaling is the mitochondrion ? a bacteria-derived organelle with its own genome, the mitochondrial DNA (mtDNA). Recent evidence has demonstrated that various stressors induce the release of mitochondria- derived signaling molecules, or mitokines. Mitokines travel systemically and modulate central physiological processes, including immune cell function and the production of pro-inflammatory cytokines. One key mitokine is circulating cell-free mtDNA (ccf-mtDNA), which arises from mitochondrial extrusion of mtDNA, is recognized by the toll-like receptor 9 (TLR-9), and leads to pro-inflammatory cytokine production in animal and human cells. Our preliminary data showed that ccf-mtDNA release, but not ccf-nuclear DNA, is acutely induced following a brief experimental psychological stress in humans. Thus, we suggest that ccf-mtDNA may be a missing link in the stress-inflammation cascade. In parallel, imaging and molecular studies on cellular systems suggests that ccf-mtDNA may be rapidly induced by primary neuroendocrine stress mediators, including glucocorticoids. This proposal will use an experimental socio-evaluative stress task, counterbalanced with a control no-stress visit for each participant, to map the kinetics of reactivity and recovery in ccf-mtDNA and other stress mediators in healthy women and men. Aim 1 will establish: i) the magnitude and kinetics of ccf- mtDNA release; ii) the specificity of this response compared to nuclear DNA; iii) examine potential sex differences in this process; and iv) statistically test ccf-mtDNA as a mediator of the subsequent increase in pro- inflammatory cytokines. In Aim 2, cellular and molecular studies will determine: i) whether primary glucocorticoid and catecholamine stress hormones are sufficient to trigger ccf-mtDNA release in primary human cultured cells and isolated leukocytes; ii) image in living cells mtDNA release in response to stress signaling; and iii) document physical changes in mtDNA structure that contribute to ccf-mtDNA release. Finally, Aim 3 will establish: i) the capacity of ccf-mtDNA from peak ccf-mtDNA serum to activate cytokine production in target cells; ii) the structural form and accessibility of ccf-mtDNA in human serum (from Aim 1); and iii) the dependency on TLR-9 for immune activation. Overall, these studies will contribute mechanistic evidence for a novel mitochondria-mediated mechanisms for transducing stress into inflammation in humans. Outcomes from this work will identify new potential targets to improve stress-related mental health and inflammatory disorders.